PROJECT SUMMARY The sickle hemoglobin mutation afflicts millions of people worldwide and is associated with considerable morbidity and mortality. The pathophysiology of Sickle Cell Disease (SCD) is a consequence of abnormal deoxygenated sickle hemoglobin polymerization and its deleterious effects on Red Blood Cell (RBC) membrane, shape, density, deformability, and adhesion. The original powerful observation that sickle red cells show abnormal adhesion to endothelial cells has since been deepened and expanded to describe a complex pathophysiology in which abnormal white blood cell (WBC) adhesion also plays an important role. These studies led to clinical trial development utilizing targeted anti-adhesion therapy. Despite the remarkable insights about abnormal cellular adhesion in SCD that have been made, there remain gaps in knowledge about these complex adhesive interactions. There is no established `atlas' of abnormal adhesive events, examined longitudinally and in a standardized manner in a large heterogeneous population of SCD patients under a range of clinical circumstance and with and without treatment. Neither the topography of adhesive events for an individual patient, nor for the SCD population as a whole is known. Better knowledge of the nature and scope of abnormal adhesive events is critical to the goals of establishing associations with clinical outcomes and successfully identifying therapeutic targets in clinical trials. We have developed a novel microfluidic assay that allows rapid, preprocessing free, and standardized interrogation of RBC and WBC adhesion in whole blood. In our ongoing experiments, high levels of HbF are associated with lesser adhesion, while greater adhesion is associated with hemolysis. In preliminary longitudinal studies, we find that RBC adhesion drops with initiation of treatment, and that levels of adhesion are stable in stably treated patients. Given our preliminary findings, we hypothesize that, in SCD: (1) changes in RBC or WBC adhesion will reflect the subjects' clinical state and treatment response, and (2) that the adhesive profile will change with age, and will differ between children and adults during vaso-occlusive crises (VOCs). To test these hypotheses, we propose the following distinct but interrelated Specific Aims: Aim 1: To standardize the simultaneous baseline evaluation of multiple cellular adhesive properties in a large population of asymptomatic adults and children (at more than one center and longitudinally); Aim 2: To determine the change from baseline in cellular adhesive properties that are present during vaso-occlusive crises, and to analyze these in adult and pediatric populations; and Aim 3: To examine changes in cellular adhesive properties before and after therapeutic interventions, including transfusions, hydroxyurea, and targeted anti- adhesion therapy. Our studies will afford the more precise characterization of abnormal adhesive events in a given individual and a more accurate assessment of response to therapy overall. We would like to make adhesion testing as feasible, and clinically meaningful, in SCD as glucose testing is in diabetes.